Alternating current motor control



Nov. 18, 1947. H. w. GRAYBROOK 2,431,242

ALTERNATING CURRENT MOTOR CUNTROL Filed Jan. 7, i944 Power Fkctar Relay 1a o 17 I l o WITNESSES: INVENTOR W Herbal! M Gragbzm. 7. BY% 2 Z ATTORNEY Patented Nov. 18, 1947 ALTEKNATING CURRENT MOTOR CONTROL Herbert W. Graybrook, Irwin, 2a., assignor to Westinghouse Electric Corporation, Eastrlittb burgh, 1a., a corporation of Pennsylvania Application January 7, 1944, Serial No. 517,314

The present invention relates to alternatlng-,

current motors which have a capacitor, or capacitors, connected in the motor circuit, and more particularly to the problem of self-excitation of such motors.

Certain types of single-phase alternating-current motors have a capacitor connected in series with an auxiliary or capacitor winding which is permanently connected in parallel with the main winding of the motor at all times during normal operation. When such a motor is disconnected from the line, it will continue to rotate for an appreciable length of time while coming to a stop, and if the proper inherent relations exist between the constants of the motor circuit, it will function as a self-excited induction generator during this period, the excitation being furnished by the capacitor. This effect is usually of no particular importance in the case of small single-phase motors, such as fractional horsepower motors, but its occurrence may have serious consequences in larger motors. Thus, in the case of a capacitor motor of relatively large size driving a highinertia load, the motor may continue to rotate for a considerable time. The self-excited generated voltage in the motor during this period may be as much as 180 out of phase with the line voltage, and if the motor is reconnected to the line while it is still rotating, severe electrical and mechanical stresses are set up in the motor which may result in serious damage, such as permanently twisting the motor shaft, or shearing off keys. A similar eifect may also occur in the case of polyphase motors with capacitors connected to the motor circuitfor power factor correction, or other purposes. The principal object of the present invention is to provide control means for alternating-current motors which have a capacitor, or capacitors, connected to the motor circuit in which operation of the motor as a selfexcited induction generator is prevented.

A more specific object of the invention is to provide control means for alternating-current motors having a capacitor, or capacitors, connected to the motor circuit in which the capacitor is disconnected from th motor circuit when the motor is .deenergized, and is reconnected to the motor circuit when the motor is again connected to the line. a

Further objects and advantages of the invention will be apparent from th following detailed description, taken in connection with the accompanying drawing, in which:

Figure 1 is a schematic wiring diagram showing one embodiment of the'invention,

2 Claims. (Cl. 318---22U 2 a Fig. 2 is a similar diagram showing another embodiment of the invention, and

Fig. 3 is a wiring diagram showing the invention applied to a three-phase motor. The invention is shown in Fig. 1 applied to a single-phase capacitor motor I having a main primary winding 2 and an auxiliary or capacitor primary winding 3, which has a capacitor 4 permanently connected in series with it. The motor I also has a rotor member 5 with a squirrel-cage secondary winding 8 onit. The construction and arrangement of'the motor I may be of any suitable or usual type, and the primary windings 2 and 3 of the motor I are connected in parallel to a single-phase supply line by a line switch I. The motor I is to be understood as; representing any type of capacitor-run motor, having either a squirrel-cage or wire-wound secondary member, and utilizing either the same value of capacitance for starting and running, or diiferent values.

In the usual construction of capacitor motors of the type described, the capacitor winding is permanently connected in parallel with the main winding, and part or all of the capacitor remains in the circuit at all times. Such a motor, nowever, may operate as aself-excited induction generator while it continues to rotate after disconnection from the line,a's described above, and is subject to the resultant danger of serious damage to the motor or associated equipment if the motor is reconnected to the line while still rotating.

In order to avoid these difficulties, in accordance with the present invention, the capacitor winding 3 and capacitor I are controlled by a contactor 8, which has a contact 9 connected in series with the capacitor 4, and an operating coil In which may be energized from any suitable source of electric energy, indicated as a battery II. The contactor 8 is controlled by means of a relay I 2 having a contact I3 connected in series with the contactor coil I0, and an operating coil II. The operating coil II of the relay I2 is energized from a current transformer IS in response to the line current flowing to the motor I.

It will be seen that when the motor I is in normal operation with the line switch I closed, the relay I2 will be energized and its contact I3 closed, so that the contactor coil I0 is energized to hold the contact 8 closed, thus connecting the capacitor winding 3 and capacitor 4 across the main winding 2. When the line switch 1 is opened to deenergize the motor, the relay I2 is also deenergized and'opens its contact I3, so that the contactor 8 is permitted to open its contact 56 8 and disconnect the capacitor 4 from the motor line while still rotating by closing the line switch I. when the switch I is closed. with the motor either stationary or rotating, the current flowing tothewinding 2 causesthe relay l2 tocloseits contact I3, thus energizing the contactor. coil ll to close the contact 8 and reeonnect the capacitorwindingSandcapacitorlinthemotorcircuit, so that the motor will operate in the normal way. Thus, complete protection is provided against self-excitation of the motor I, and the danger c: severe mechanical and electrical streses occurring in the motor if it is reconnected to the line while still rotating is avoided.

Fig. 2 shows an alternative embodiment of the invention in which the contactor I is controlled by a power-factor relay ii. The relay Ii may be of any suitable type, and has been shown as having a current coil I1 and a voltage coil it connected in series with and across the main winding 2, respectively. The relay It has a contact 19 which is closed when the voltage and current are in the normal phase relation which exists during operation of the motor. when the line switch I is opened to deenergize the motor, a generated voltage appears in the motor windings which is reversed in phase position from the previously applied voltage of the Supply line. This reversal in phase position of the voltage across the main winding 2 causes the relay ii to open its contact 19, and thus to deenergize the contactor coil it so that the capacitor winding 3 and capacitor I are disconnected from the motor circuit. When the line switch I is closed to reconnect the motor to the line, the relation of the applied voltage and the current is such that the relay it closes its contact i3, and causes the contaetor 8 to reconnect the capacitor winding 3 and the capacitor 4 in the motor circuit. It will be apparent that the general operation and advantages of this embodiment of the invention are the same as those of the embodiment shown in Fig. l, and that the embodiment of Fig. 2 diifers only in usim; a power-factor-responsive relay rather than a cln'rent-responsive relay.

It is fairly common practice to connect capacitors across the windings of polyphase motors to improve the power factor, or for other reasons, and when such capacitors are connected directly across the motor windings, as is often done in the case of motors in industrial plants, for example, the same danger of self-excitation of the motor upon disconnection from the line exists as discussed above in connection with single-phase mtors, and the present invention is also applicable to the prevention of this eifect in polyphase motors.

Fig. 3 shows the invention applied to a threephase induction motor 2! having three-phase primary windings 2i and a squirrel-cage secondary winding 22 carried on a rotor member 23. The motor 28 may be any usual type of polyphase electric motor, although a squirrel-cage induction motor has been shownfor the P pose of illustration. The motor 2| is connected to a threephase supp y line by means of a line switch 2|, and a capacitor 15 is connedzed across each phase of the motor This is a common arrangement for-improving the power factor. In order to prevent the posibility of self-excitation of the motor Zlwhenitisdisconnectedfromtheline,

v 4 eachofthecapadtorsliisccnn-olledbyacmtactor iihavingacmtact 21 connected inner-lea with the capacitor. Each of the eontadora II hasanoperatingcoilllandtheoperatingcdll jlofallthecontactorsarearrahgedtobesimultaneouswenergizedfromanymitablesom'ceot electricenergmsuohasabatterya. Asshown inthedrawingailthreecoilsllareumnected in series and their energlzation is controlled by thecontactllofarelayil. Itwillbeunderstood, ofcourse,'thatthecontactorcoils2lcouldequally well be connected in parallel, the essential requirement being that they shall be substantially simultaneously energized and deenergiaed. It willalsobeapparentthatasinglecontactorcould beusedhavingseparatecontactsforeachofthc capacitors 25. The relay 3| has acoil "which issuppliedfromacurrenttnnsformerllconnectedinseriesinoneofthethree-phaseleuh supplying the motor 2|.

It will be apparent that the operation of the embodiment of the invention shown in Fig. 3 is suhstantiallythesameasthatofthecimiitot.

Fig. 1. Thus, in normal operation or the meta, theoontactiloftherelayilisheldclosedby reason of the current flow to the motor which energizes the coil 32 through the current tramfonner 33- The coils II of the cont-actors 28 are thus connected to the power source I! and hold theircontactstlclosedsothatthecapaeitorsli are connected across the motor windings II. when the switch'll is opened to deenerglae the motor 2', the flow of current through the transformer 33 is interrupted and the relay contact 3| opens, deenergizing the contactor coils 28 and permitting the contactors 26 to open their contacts 21 and disconnect the capacitors 25 from the motor. Thus, the possibility of self-excitation of the motor II is prevented, and the switch 24 can be reclosed while the motor is still rotating without danger of any damage to the motor resulting. Upon reclosure of the switch 24, the relay II recloses its contact 3|, causing the cont-actors II to again connect the capacitors 25 to the drcuit.

It should now be apparent that control means have been provided for alternating-current motors, either single-phas or polyphase, which have a capacitor, or capacitors, connected to the motor circuit, which positively prevents the possibility of self-excitation of the motor while it is still rotating after being disconnected from the line, andthusthedangerofeithermechanicalorelectricai damage to the motor is avoided. It is to be understood that the invention is capable of various modifications and embodiments and that the three specific embodiments shown are only illustrative. Thus, in some cases where the currents involved are not too great, the contacts of therelays I! or limightbeconnecteddireetly in the capacitor circuit instead or controlling a contactor. fimilarly, the arrangement and cmnectionsoftherelaysmaybevariedandother typzsofrelaysmightbeusedforthepurposeot automatically effecting disconnection of the capacitor in response to deenergization of the motor.

It will be apparent, therefore, that the invention is capable of various modifications and embodiments within the scope of the broad principla of the invention, and it is to be understood that although certain specific embodiments have been shown and dmcribed for purposes of illustratlm, the invention is not limited to these particular embodiments but in its broadest aspects it includes all equivalent embodiments and modifications which come within the scope of the appended claims.

I claim as my invention: 1. A single-phase alternating current electric motor having a main primary winding and an i auxiliary primary windin a capacitor, means for connecting said capacitor in series with said auxiliary primary winding at all times during normal operation of the motor, and means responsive to the phase relation between motor current and motor voltage for effecting disconnection of the capacitor from the motor in response to a reversal in phase position of said voltage.

2. A single-phase alternating current electric motor having a main primary winding and an auxiliary primary winding, a capacitor, means for connecting said capacitor in series with said auxiliary primary winding at all times during normal operation of the motor, and relay means responsive to the phase relation between motor current and motor voltage for efiecting disconnection of the capacitor from the motor in response to reversal in the relative phase position of said current and voltage.

HERBERT W. GRAYBROOK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

